Measurement of crystallinity of polyamide-6 by d.s.c.

Notes to the E d i t o r

(ppm) N.m.r.spectrum of polyether (IV) in CS2

Figure 1

water. The yield obtained was 82% of the theoretical value, being 2.58 g. Details of the measurement of the n.m.r, spectrum and the gel chromatogram are provided in an earlier publication 7. REFERENCES

I

250 '

,oo

V¢ (mL)

Gel permeation chromatogram of polyether IV (Merckogel 5000/THF) Figure 2

1 Clark, R. H. and Streight, H. R. L. Proc. Trans. R. Soc. Can. Sect.3 1929, 23, 77;Gomberg, M. Ber. Dtsch. Chem. Ges. 1902, 35, 2401 2 Braun, D. and Platzek, U. Makromol. Chem. 1973, 164, 55 3 Hine, J. 'Reaktivitat und Mechanismus in der organischen Chemie', G. Thieme, Stuttgart, 1960, p 113 4 Carrd, P. and Libermann, D. Bull. Soc. Chim. Fr. 1933, 53, 1050; CarrY, P. C. R. Acad. Sci. 1933, 196, 1409, 1806; 1934, 198,274 5 Moravec,J., Kourim, P. and Horak, M. Coll. Czech. Chem. Commun. 1965, 30, 2298 6 Funakubo, E. and Matsui, T. Ber. Dtsch. Chem. Ges. 1937, 70, 2437 7 Braun, D. and Platzek, U. Makromol. Chem. 1973, 164, 41

Measurement of crystallinity of polyamide-6 by d.s.c. G. Coppola, R. Filippini and B. Pallesi Snia Viscosa, Centro Sperirnentale F. Marinotti, 20031 Cesano Maderno, Milano, Italy (Received 23 December 1974)

INTRODUCTION

EXPERIMENTAL

The aim of this work was to provide a calibration plot for routine measurements of the degree o f crystallinity of commercial polyamide-6 through enthalpies o f fusion, 2d-/f, derived by differential scanning calorimetry (d.s.c.).

Some commercial samples o f polyamide-6 were moulded at different temperatures to films of 1 mm thickness, thus achieving various degrees of crystallinity (up to 50%). One totally amorphous sample was prepared by quenching in

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POLYMER, 1975, Vol 16, July

Notes to the E d i t o r

liquid nitrogen (-196°C) after melting at 250°C. This sample did not show any crystallinity by observation under a polarizing microscope. Degrees of crystallinity were calculated from the density values measured by a gradient column, using the well known equation: c-

dx

da

2C -6 U

<~ IO

100dc

dc - da

30

(1)

dx

were C is the percentage crystallinity, dx, da and d c are the densities of the sample, of the totally amorphous and totally crystalline polymer, respectively. The density of the quenched sample was 1.098 g/cm 3, in good agreement with published data 1. A/-/f values were measured, on samples of 20 mg each, by a Du Pont Model 900 thermal analyser equipped with a d.s.c, cell, at a temperature scanning speed of 30°C/min and under a nitrogen atmosphere. These conditions have been adopted in order to minimize sample crystallization during heating. For samples of low crystallinity, however, crystallization above Tg (glass temperature) is unavoidable; in these cases, the exothermic peak was always subtracted from the melting area. RESULTS AND DISCUSSION In order to derive C values from equation (1) we tentatively substitute the value of d c = i .220 g/cm3; this is an average value o f various published data 2-4 which are not in good agreement, however, since they range from 1.243 to 1.133 g/cm 3. By plotting AHf against C, we obtain, nevertheless, not one but two straight lines; subsequent analysis by X-rays s showed that samples rich in a crystalline form gave one line, distinct from the other on which samples rich in 7 form were allocated. This finding is in complete agreement with Illers' conclusions< Since our object was to produce a simple calibration plot of C vs. 2ff/f to be used for samples (films, monofflaments, etc.) where density measurements are difficult, the existence of two lines complicates the whole matter, requiring X-ray analysis o f each sample in order to determine the crystalline form. We have therefore plotted in Figure I only the values concerning those samples in which one of the two forms (a or 7) was prevailing; in equation (1) two d c values were used: 1.235 for a crystals and 1.190 for 7 crystals, which are averages of published data 4-6. By the least squares method we calculated the single

o

,6

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2%

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3'0

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s'o

c (O/o) Figure 2 Corrected calibration curve considering the amorphous crystallization after Tg

upper line shown in Figure 1. We assumed that 2 f f / [ values for a alad 3' forms are practically identical; with this assumption the plot is valid whatever the crystal composition of any sample. Figure I shows that the calibration line does not pass through the origin but cuts the ordinate axis at a value of 2ff-/f = 5.20 cal/g. This same observation was made by lllers 1, who explained it by assuming the existence of some ordered structures in the amorphous phase. This is probably true for their samples, which were quenched, after melting, in carbon tetrachloride at 0°C. Our samples, quenched at a much lower temperature, suggest a better explanation, i.e. to assume a slow crystallization of the sample during heating in the calorimeter. Polyamides-6 can easily crystallize owing to their low Tg values. Crystallization is known to occur immediately above Tg and is put into evidence by an exothermic peak but the phenomenon probably takes place even after this peak, until the polymer melts; this is not easily shown being included in the base line. This effect produces values of ~J-/f which are 2 0 - 3 0 % higher than those expected from the crystallinity of the original samples. Since, however, M-//values have been correlated with degrees of crystallinity measured by density values, the plot of Figure 1 is still good for the purpose we wanted. As an indication of the correctness of our hypothesis, we can assume that the amount of crystallinity introduced into the sample on heating is proportional to the original amorphous content; by multiplying this value for 5.2 cal/g and subtracting this quantity from the measured 2d-/f, we obtain a new line, shown in Figure 2, which passes through the origin.

ACKNOWLEDGEMENT 30

We are particularly grateful to Professor U. Bianchi (Istituto di Chimica Industriale dell'Universit~i di Genova) for critical reading of the manuscript.

c. 2 0

REFERENCES

I

0

I

I0

I

I

20

I

I

I

I

I

I

30 40 50 C (°/o) Figure 1 Calibration curve: values concern samples having only c~or 3' forms

1 Illers,K. H. and Habernkorn, H. Makromol. Chem. 1971,142, 31 2 Holmes, D. R., Bunn, C. W. and Smith, D. I. J. Polym. Sci. 1955, 17, 159 3 Ruscher, C. and SchrOder, H. J. Faserforsch. Textiltech. 1960, 11,165 4 Roldan, L. G. and Kaufman, H. S. J. Polym. Sci. (B} 1963, 1, 603 5 D'AI6, B., Coppola, G. and Pallesi, B. Polymer 1974, 15, 130

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